The invention herein pertains generally to receivers for receiving communications transmitted by way of a carrier signal having a prescribed frequency and specifically to such receivers which employ frequency synthesizers responsive to digital signals for providing the injection signals of appropriate frequency with which to demodulate the carrier.
Frequency synthesizers, which take advantage of digital electronic techniques, have widely supplanted the older types of analog A.C. signal sources for providing the injection signals required by radio receivers to demodulate the carrier signal over which communications are transmitted. In adapting this popular component to sweep-tune receivers, namely, receivers which permit facilely and conveniently tuning across a wide range of frequencies for possible transmissions over discrete frequencies within the frequency range, problems were encountered because of the conventional push button tuning control associated with frequency synthesizers vis-a-vis, the older mechanical rotary knob which could be rapidly rotated to change the value of reactive components which comprise the earlier injection signal sources to quickly vary the frequency smoothly and continuously when tuning. One solution to this problem, which was patented in U.S. Pat. No. 3,665,323, entitled "Proportional Digital Control For Radio Frequency Synthesizers" issued to Max Peterson and having the same assignee as the subject patent application, entails the use of a rotary knob for controlling a frequency synthesizer through the generation of a pair of pulse trains from an optical encoder whereby each rotation of the knob gives rise to a predetermined number of pulses to drive a digital counter whose digital output specifies the frequency of the frequency synthesizer. The relative phase of the trains is controlled by the direction of rotation of the knob for counting either up or down so as to either raise or lower the frequency, dependent on whether the knob rotation is clockwise or counterclockwise.
The foregoing arrangement is acceptable when the frequency synthesizer and tuning equipment, including the rotary knob tuning mechanism, are colocated or at least in close proximity to one another. However, when these are quite distant, as may often be the case where the economics preclude manning a sizeable number of sweep-tune monitor receivers, which one government may scatter over a wide region for intercepting messages of other foreign governments, the control of a receiver is effectuated remotely from a central manned location which may be hundreds or even thousands of miles distant, necessitating the use of communications media. These media, such as telephone lines, impose a constraint on the transmission rate of data so that it may not be possible for the digital counter associated with the frequency synthesizer to continuously and accurately receive and therefore track the data embodying the pulse train information at the control point, particularly if the rotary knob which gives rise to the pulses is being rapidly rotated. For example, when the pulses are converted to a conventional binary coded decimal format requiring four bits per individual digit specifying frequency, as taught in the previously mentioned Peterson patent, and the frequency specification extends from 1 hertz to 100 megahertz entailing eight frequency digits, the number of bits necessary to transmit the absolute frequency to which the frequency synthesizer is to be tuned is 32, not even considering the other required bits such as for timing, framing and parity check. Even if one were not to transmit the absolute frequency, but say the incremental frequency which might extend up to 10 kilohertz for rapid tuning, entailing five frequency digits, still 20 bits of information would be required for each frequency adjustment, ignoring the other requisite data bits. Transmitting the data over a communications link limited to 1200 bits/sec, as commonly found on voice grade telephone lines, would permit less than 60 frequency adjustments per second assuming 20 bits per adjustment. Compare this with the number of frequency adjustments possible by the operation of the rotary knob which typically produces 90 pulses and 90 corresponding frequency adjustments per knob rotation. At a rotation rate of three revolutions/second, which is not unreasonable, a human operator can produce 270 frequency changes per second. Were each frequency change to be encoded into a 20 bit digital word and transmitted over a 1200 bits/sec link, permitting only 60 frequency adjustments per second, some 210 discrete frequency changes could not be tracked and therefore monitored. Thus, whereas the direct application of pulses generated by the rotary knob to a digital counter as described in the aforementioned patent for local control results in acceptable operation, it is apparent that transmitting frequency data in an absolute or differential format over communication links results in lost data so that the frequency synthesizer to be controlled thereby does not properly track the remote control operation. This is unacceptable.
Another problem intrinsic to the sweep tuning digital technique described in the aforementioned patent is the linear relationship between the synthesizer tuning rate and the rotary knob operation. Since there are a fixed number of pulses generated for each knob revolution and the up/down counter for controlling the frequency synthesizer is merely clocked from these pulses, the counting rate which determines the speed with which the synthesizer is tuned is directly proportional to how fast the operator turns the rotary knob. Although, during the course of tuning the operator can select which frequency decade to change when traversing a particular frequency band, there is no flexibility within the chosen frequency band to tailor the synthesizer tuning rate to the rotary knob rotational speed, for example in exponential fashion, so as to enhance the facility with which an operator can traverse a wide range of frequencies.
In devising a scheme for translating the movement of a manually operated tuning mechanism, such as a rotary knob, into digital signals for controlling a frequency synthesizer, it is important and in some cases even essential that the scheme engender processor compatibility since many monitor receivers are offered with processor control systems. When in a processor control mode, the digital signals for controlling the frequency synthesizer are derived not from the tuning mechanism but rather from the digital output signal from the processor in accordance with the programmed control.
With the foregoing in mind, it is a primary object of the present invention to provide a new and improved means for digitally sweep-tuning a frequency synthesizer.
It is a further object of the present invention to provide such a new and improved means that engenders a minimal amount of data so as to lend itself to transmission over communication paths having data rate constraints.
It is still a further object of the present invention to provide such a new and improved means which affords programmability for tailoring different tuning rates to different control rates.
It is still a further object of the present invention to provide such a new and improved means that is compatible with both manual and processor controls.
The foregoing objects, as well as others, and the means by which they are achieved through the present invention, may best be appreciated by referring to the Detailed Description of the Preferred Embodiment which follows together with the appended drawings.